Silver alloy reflective film materials, as reflective films typically for optical discs (hereinafter also referred to as “optical information storage media”), show superior properties than other materials, such as high reflectivity, high transmittance, low absorptivity [absorptivity=100%−(reflectivity+transmittance)], and high thermal conductivity.
However, it has been an important technological issue to improve long-term reliability in optical discs using these silver alloy reflective films, so as to maintain these superior properties over the long term. The long-term reliability is most affected by environmental resistance, such as hygrothermal resistance and light stability, of silver alloy reflective films, and thus the environmental resistance should be improved.
Typically, when an optical disc is used in higher temperatures and humidity surroundings, silver (Ag) becomes more liable to migrate (diffuse) and aggregate from the silver alloy reflective film to a resin layer arranged in contact with the reflective film. The diffusion and aggregation of silver causes increased surface roughness and/or loss of the continuity of the silver alloy reflective film to reduce the reflectivity, and this remarkably impedes functions as a reflective film or semi-reflective film.
When an optical disc is used under such conditions as to be irradiated with ultraviolet rays are applied typically from fluorescent lamps, the light irradiation also makes silver more liable to migrate (diffuse) and aggregate from the silver alloy reflective film to a resin layer arranged in contact with the reflective film. The diffusion and aggregation of silver in turn causes reduction in reflectivity, and at the time when the reduced reflectivity reaches the lower limit of reflectivity for the detection of regenerative signals, it becomes difficult to regenerate signals.
Various techniques have been proposed for improving properties of reflective films by allowing silver with specific rare-earth elements or by improvements of silver alloys. Typically, there have been proposed silver alloy reflective films containing, for example, Ag—Cu—Au—(Nd, Sn, Ge) and silver alloy reflective films containing Ag—(Bi, Sb)—(Cu, Au), or Ag—(Bi, Sb)-(rare earth element: Nd, Y)—(Cu, Au) (see Patent Documents 1 and 2).
There has been also proposed a technique of reducing the thermal conductivity of a silver alloy in a reflective film, so as to carry out laser marking at a lower power. Specifically, Ge, Si, Sn, Pb, Ga, In, Tl, Sb, and/or Bi is added to silver (Ag) to reduce the thermal conductivity (see Patent Document 3). Likewise, there has also been proposed a technique of adding Cr, Ti, Si, Ta, Nb, Pt, Ir, Fe, Re, Sb, Zr, Sn, and/or Ni to silver to reduce the thermal conductivity (see Patent Document 4).
These reflective films composed of known silver alloys, however, are not intended to improve environmental resistance such as hygrothermal resistance and light stability. Additionally, such reflective films composed of known silver alloys are liable to deteriorate in the environmental resistance when the metal reflective films (silver alloy reflective films) are each in direct contact with an ultraviolet-curable resin layer or organic dye recording layer in optical information storage media, as described below.
As a possible solution to avoid reduction in environmental resistance of a silver alloy reflective film when the metal (silver alloy) reflective film is in direct contact with an ultraviolet-curable resin layer or organic dye recording layer, a Li-containing silver alloy reflective film has been proposed (Patent Document 5). More specifically, this document proposes a silver alloy containing 0.01 to 10 atomic percent of Li. Additionally, this document discloses that the silver alloy may further selectively contain 0.005 to 0.8 atomic percent of Bi; a total of 0.1 to 2 atomic percent of rare-earth metal elements (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu); and/or a total of 0.1 to 3 atomic percent of one or more element selected from Cu, Au, Rh, Pd, and Pt.    [Patent Document 1] Japanese Unexamined Patent Application Publication (JP-A) No. 2002-15464    [Patent Document 2] JP-A No. 2004-139712    [Patent Document 3] JP-A No. 1992-252440    [Patent Document 4] JP-A No. 1992-28032    [Patent Document 5] JP-A No. 2006-48899